In order to evaluate visual function in infants and young children one must first capture and maintain their visual attention. For many children, especially those with visual impairments or multiple handicaps, behavioral tests such as preferential looking (PL) must employ more attractive and dynamic stimuli, and more flexible and efficient testing procedures than those presently available. Two modifications - one electronic-one optical - of a PL apparatus are proposed in response to these demands on the technique. A computerized, video version of FPL (76,77) under development in our laboratory provides a variety of stimuli for display on either of the two monitors which replace previous photographic stimuli. Computer control of both the stimulus display and data storage makes it possible for one adult to test and infant. Our purpose is to further develop, and to validate this version against standard FPL techniques. The second, even more promising, modification is of particular clinical relevance. During early infancy, clinicians seek to diagnose and treat those ocular, refractive, or oculomotor disorders believed to place infants at risk for amblyopia. To monitor the interocular differences in visual function which characterize amblyopia, monocular measures are essential. Such data are, however, more difficult to obtain than binocular measures, especially in infants who already show a strong monocular preference. Guyton (34,35,36) developed an optical system which permits 'remote' occlusion, and demonstrated convincingly its usefulness in performing cover tests. Incorporation of his system into an FPL apparatus would allow brief, non-disruptive, alternate occlusion of infants during vision testing. For use with FPL, it must be modified to reduce the viewing distance and to enable simultaneous, monocular occlusion for both positions in the stimulus display. Comparisons of monocular and binocular data are proposed. An FPL apparatus which includes the electronic and optical modules proposed here would constitute a powerful, yet flexible instrument for non-invasive assessment of infant vision. Procedural efficiency, stimulus enhancement, and reduced disruption due to occlusion are expected to overcome some infants' resistance to testing, and to yield more data of better quality.